This invention relates generally to a wand for handling a work piece, and more specifically, in one embodiment, to a work piece handling wand for efficiently loading and unloading work pieces into and from a processing apparatus and to a method for processing work pieces that involves the use of such a wand.
Many manufacturing processes require the automated loading and unloading of work pieces into and out of a processing apparatus. For example, work pieces may be moved sequentially from one piece of processing apparatus or from a work piece storage site such as a work piece cache into a second work piece processing apparatus. After being processed in the second work piece processing apparatus, each work piece may then be transferred to yet another piece of processing apparatus or to another work piece cache. In the interest of reducing cost and increasing productivity, such movement of work pieces is often accomplished with the aid of an end effector or work piece handling wand on the end of a robotic arm. The manner in which the work pieces are contacted by the wand and the accuracy of the placement of the work pieces is often of critical importance to the yield of the process.
An example of one such manufacturing process is the planarization of a surface of a work piece, a process that finds application in the manufacture of many types of products. Examples of work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. Chemical mechanical planarization (CMP) is one accepted method of achieving a planar surface. The CMP method typically requires the work piece to be loaded into and mounted on a carrier head in a manner such that the surface that is to be planarized is exposed. The exposed side of the work piece is then held against a polishing pad and a relative motion is initiated between the work piece surface and the polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, is introduced at the interface between the work piece surface and the pad.
Many types of work pieces such as the semiconductor wafers on which integrated circuits and other semiconductor devices are manufactured are typically processed in batches or lots that include a plurality of work pieces. For example, with the CMP processing of semiconductor wafers, each of the wafers in a lot must be sequentially loaded from a wafer cache onto the carrier head for planarization. Following the planarization, each wafer is unloaded from the carrier head and again placed in a wafer cache or is directly transferred to a subsequent processing apparatus such as a cleaning station. The loading and unloading operations are accomplished using a wafer handling wand.
With many work pieces, and certainly with semiconductor device wafers, the surfaces of the work pieces can be easily damaged if the handling wand contacts the surface. Because of this, the wand should preferably contact only the edge of the work piece or, or at most, the surface within a narrow distance from the edge.
Loading a work piece into a conventional carrier head for performing chemical mechanical planarization of that work piece presents problems for conventional work piece handling wands. The conventional carrier head includes a circular work piece carrier plate having a diameter substantially equal to the diameter of the work piece to be polished. An annular wear ring composed of a hard material such as ceramic is rigidly mounted to the main carrier head housing immediately radially outward of the rigid carrier plate. The wear ring has a diameter just slightly larger than the diameter of the work piece. To carry out the planarization operation, the work piece must be mounted against the carrier plate within the confines of the wear ring.
In the CMP process the carrier head with a work piece mounted thereon is pressed against a polishing pad so that the wear ring and the surface of the work piece are pressed against the pad. The pad then is set into motion to begin the polishing process. The work piece is maintained on center with respect to the carrier head by the cylindrical inside surface of the wear ring which traps the work piece between the polishing pad and the carrier plate. In order to maintain the precise control of the position of the work piece necessary for CMP, the clearance between the outside diameter of the work piece and the inside diameter of the wear ring must be kept to a minimum. For example, in the CMP processing of a 200 mm (8 inch) semiconductor wafer the clearance is typically less than 1 mm. This tight tolerance between the work piece and the wear ring poses substantial design challenges in terms of the necessity for highly accurate positioning of the work piece prior to loading the work piece in the carrier head. To accomplish a reliable loading of the carrier head, given the tight tolerance restrictions, the robotic arm must be able to pick up a work piece from the previous station and place it in the carrier head with a combined positional accuracy of less than 0.5 mm. A work piece that is out of position at the previous station when it is picked up by the robot arm and/or a robot tool point position that is out of position at the carrier head by more than a combined 0.5 mm will cause the work piece to be misloaded, leading to a stoppage in the throughput of the CMP apparatus. Worse, if a failure in the loading of the carrier head is not detected, the work piece can slip between the wear ring and polishing pad, resulting in a total loss of the work piece. In addition, because of the fragile nature of the surfaces of the work piece, it is desirable that the work piece be contacted only on its edges, and this complicates the task of inserting the work piece into the recess formed by the wear ring.
Many other types of processing apparatus also require that the work piece be aligned within tight tolerances during the loading operation. Although robotic handling equipment can generally perform repeated operations with great precision, some alignment aid is still necessary to account for manufacturing tolerances, wear, run out, and the like.
Accordingly, there exists a need for a work piece handling wand that can load work pieces into and unload work pieces from a work piece processing apparatus with a high degree of precision and without adversely contacting the critical surfaces of the work piece. There also exists a need for a method for processing work pieces in an efficient manner using such a work piece handling wand.